1. Field of the Invention
This invention relates to a doublet lens, a variable apex angle prism and a deviation correction device, usable with advantage for correction of optical deflection in, for example, a camera device.
2. Description of the Related Art
Recently, for suppressing oscillations of an imaged picture by oscillations transmitted to a camera during imaging by a television camera, there is marketed a device comprised of an acceleration sensor in a camera device for detecting the an angular position of the camera device and the apex angle of the prism placed in the light path of the imaging camera lens is varied responsive to an output of the sensor for canceling the oscillations of the image due to the oscillations of the camera device by tilting the optical axis. For example, there is disclosed in Japanese Laying-Open Patent Publication 61-269572 a deviation correction device in which a special liquid is sealed on an optical axis of the lens by being sandwiched between two glass plates and a bellows interconnecting the two glass plates and in which the angular position of one of the glass plates is changed for varying the apex angle of the prism for correcting the optical axis by an angle corresponding to the angular deviation of the camera device.
There is also marketed a concave-convex lens variable-apex-angle prism, as a deviation correction optical system, in which a plano-convex lens and a plano-concave lens having equal refractive index and curvature values are set with the spherical surfaces thereof facing each other with a small gap in-between and in which one of the lenses is rotated along the spherical surface for varying the angle of two planar surfaces facing each other with the interposition of the spherical surfaces. In Japanese Laying-Open Patent Publication 6-281889, there is disclosed a deviation correcting optical system employing a variable prism comprised of the combination of convex and concave lenses.
However, in the above-mentioned variable prism of the liquid sealing type, since the liquid is sealed in a space defined between two glass plates and the bellows interconnecting the glass plates, the liquid operates as viscous resistance in case the glass plate angle is changed to render it difficult to follow up with high-speed deviations.
On the other hand, the variable apex angle prism by the combination of the convex and concave lenses has not been used to date extensively. The reason is that the curvature of the concave and convex lenses cannot be reduced sufficiently and hence the prism cannot be held for rotation about the center of curvature in a limited space, and that, if the prism is slid on a spherical surface having the same center as the concave and convex lenses, the prism is apt to be deteriorated in responsiveness due to slip or be damaged thereat.
In actuality, the method for rotationally supporting the convex lens as disclosed in the Japanese Laying-Open Patent Publication 6-281889 resides in arranging a shaft passing through the center of curvature of the lens parallel to the imaging plane outside of the lenses and the lenses are held by an arm rotatably formed on the shaft for rotationally supporting the lenses.
With this method, however, the combinatory concave and convex lenses cannot be rotated in contact with each other in view of reducing the resistance. On the other hand, the system of sealing the liquid having substantially the same refractive index as that of two glass plates between the lenses cannot be used because of increased viscous resistance. Thus, a gap needs to be formed as long as the two lenses face each other with the convex and concave surfaces thereof facing each other.
Thus the variable apex angle prism comprised of a combination of the plano-concave lens and a plano-convex lens facing each other with a gap in-between ceases to be afocal (infinite light collection type) by the combination of two lenses despite the fact that the spherical surfaces facing each other are of the same radius.
This poses no problem if the variable apex angle prism is built into an imaging optical system lying ahead and at back of the imaging optical system without the necessity of dismounting. However, if the variable apex angle-prism is mounted as when a deflection follow-up mechanism is added while the prism can be dismounted from a pre-existing lens, such an inconvenience arises that the overall flange back or the focal length be changed.